1. Field of the Invention
This invention relates to RAID (Redundant Array of Inexpensive (or Independent) Disks (or Devices)) systems, and in particular, to method and apparatus for converting between different species of RAID's and rule-based RAID reconfiguration.
2. Description of the Related Art
RAID is a data storage system that provides a certain level of redundancy so that a certain number of disks (devices) of the disk (device) array may be lost without any loss of user data stored thereon. Various species of RAID systems are known, including RAID0, RAID1, RAID3 and RAID5 (known as standard RAID), and RAID2, RAID4 and RAID6 (known as non-standard RAID). Methods and apparatus that provide conversion or migration between different conventional RAID species have been described. For example, U.S. Pat. No. 6,275,898 describes converting from RAID5 to RAID1 (a contraction, or reduction of the usable capacity of the system, referred to as “promotion” in that patent) and converting from RAID1 to RAID5 (an expansion, or increase of the usable capacity of the system, referred to as “demotion” in that patent). The conversion must be done off line, i.e. the system cannot take user request while performing the conversion. In the context of this patent “RAID1” includes the compound RAID, which we call “RAID10”. U.S. Pat. No. 6,154,853 describes a special case of an “even” conversion (where the usable capacity in the system is unchanged), by converting an n-disk RAID5 to a 2(n-1) disk RAID10 and back. U.S. Pat. No. 5,524,204 and U.S. Pat. No. 5,615,352 describe a method for expanding a RAID5 to a bigger RAID5 with a larger number of disks. The conversion may be accomplished without interrupting service, i.e. while the system is online. These two patents do not describe an array contraction.